It is customary practice, especially in solar heating applications, to circulate a first liquid through a solar panel or boiler where it will be heated, and then to conduct the first liquid to a heat exchanger, usually disposed in a storage tank where the heat is transferred to a second liquid such as domestic water. A problem with such a system is that pollution of the domestic water supply by the heating liquid is not tolerable, and it is customary to require that there be a double wall separation between them so that if one wall fails, the other wall will still protect the domestic water supply. This is a complicated, expensive, and potentially inefficient technique.
It is an object of this invention to avoid this double wall construction by providing a safety valve which will dump the first liquid unless the pressure of the second liquid is sufficiently higher than the pressure of the first liquid so that as between the circuits, the first liquid cannot contaminate the second liquid.